1. Field of the Invention
The present invention relates to a sheet feeding apparatus applied to an image forming system such as a copying machine, printer and the like, and more particularly, it relates to a sheet feeding apparatus which feeds a sheet material by means of a plurality of rollers or an elongated roller.
2. Related Background Art
In the past, as a sheet feeding apparatus of this kind, an apparatus as shown in FIG. 6 has already been known. This conventional sheet feeding apparatus comprises three rubber feed rollers 101, 102 and 103 having the same diameter and attached to a common driving shaft 100 and can feed a sheet material S.sub.o in a direction shown by the arrow A.sub.o by rotating the driving shaft 100 by a motor (not shown). Further, in this conventional sheet feeding apparatus, a lateral registering plate 104 is arranged on one side of a sheet feeding path upstream of the driving shaft to prevent the skew-feed of the sheet material S.sub.o. In this way, the sheet material S.sub.o is prevented from shifting to the lateral direction by means of the lateral registering plate 104 and is fed by the rotational movement of the feed rollers 101-103 toward a registering roller 105 at that atitude. Such sheet feeding apparatus is also disclosed in the U.S. Pat. No. 4,098,551.
However, in such conventional sheet feeding apparatus, when the rubber feed rollers 101-103 are used for a long time, the diameters of these rollers change from each other due to uneven wear, with the result that there arises the difference in feeding speeds of the sheet material S.sub.o at the respective feed rollers 101-103.
For example, if the feed roller 103 remote from the lateral registering plate 104 wears more than the feed roller 101 near the lateral registering plate 104, the outer diameter of the feed roller 101 near the lateral registering plate 104 will be larger than that of the feed roller 103, and accordingly, the feeding speed of the feed roller 101 will also be faster than that of the feed roller 103. Accordingly, the leading portion of the sheet material S.sub.o is subject to a force directing to a direction P.sub.1, whereas the trailing portion of the sheet material is subject to a force directing to a direction P.sub.2. In this case, however, since the sheet material S.sub.o is regulated by the lateral registering plate 104, the sheet material is not skew-fed at an upstream side of the feed rollers 101-103. Further, since the leading edge of the sheet material S.sub.o is pinched by the registering roller 105 before the trailing edge of the sheet material leaves the lateral registering plate 104, the sheet material is also not skew-fed at a downstream side of the feed rollers.
To the contrary, if the feed roller 101 near the lateral registering plate 104 wears more than the feed roller 103 remote from the lateral registering plate 104, the leading portion of the sheet material S.sub.o will be subject to a force directing to a direction P.sub.3 and the trailing portion of the sheet material will be subject to a force directing to a direction P.sub.4 for the same reason as mentioned above. In this case, since there is no lateral registering plate (for preventing the skew-feed of the sheet material S.sub.o) at the opposite side of the sheet feeding path, the sheet material S.sub.o will be gradually skew-fed in the direction P.sub.4.
As mentioned above, in the conventional sheet feeding apparatus, since the skew-feed of the sheet material was caused by the uneven wear of the feed rollers, even if the feed rollers could still be used, the feed rollers had to be replaced by new ones a little earlier, thus forcing the excessive labor upon an operator.